Diamond no longer nature's hardest material

This article was first published on 16 February 2009. See the end of the article for an update

Diamond will always be a girl's best friend, but it may soon lose favour with industrial drillers.

The gemstone lost its title of the "world's hardest material" to man-made nanomaterials some time ago. Now a rare natural substance looks likely to leave them all far behind – at 58 per cent harder than diamond.

Zicheng Pan at Shanghai Jiao Tong University in China and colleagues simulated how atoms in two substances believed to have promise as very hard materials would respond to the stress of a finely tipped probe pushing down on them.

Extreme conditions

The second, the mineral lonsdaleite, or hexagonal diamond is made from carbon atoms just like diamond, but they are arranged in a different shape.

Only small amounts of wurtzite boron nitride and lonsdaleite exist naturally or have been made in the lab, so until now no one had realised their superior strength. The simulation showed that wurtzide boron nitride would withstand 18 per cent more stress than diamond, and lonsdaleite 58 per cent more. If the results are confirmed with physical experiments, both materials would be far harder than any substance ever measured.

Doing those tests won't be easy, though. Because both are rare in nature, a way is needed to make enough of either of them to test the prediction.

Rare mineral lonsdaleite is sometimes formed when meteorites containing graphite hit Earth, while wurtzite boron nitride is formed during volcanic eruptions that produce very high temperatures and pressures.

Flexible friend

If confirmed, however, wurtzite boron nitride may turn out most useful of the two, because it is stable in oxygen at higher temperatures than diamond. This makes it ideal to place on the tips of cutting and drilling tools operating at high temperatures, or as corrosion-resistant films on the surface of a space vehicle, for example.

Paradoxically, wurtzite boron nitride's hardness appears to come from the flexibility of the bonds between the atoms that make it up. When the material is stressed some bonds re-orientate themselves by about 90 degrees to relieve the tension.

Although diamond undergoes a similar process, something about the structure of wurtzite boron nitride makes it nearly 80 per cent stronger after the process takes place, says study co-author Changfeng Chen at the University of Nevada, Las Vegas, an ability diamond does not have.

Single crystals

"This is important because any attempt to give an insight into the mechanism that improves a material's property, especially hardness, is technologically extremely significant," she told New Scientist.

The more that is understood about what influences the hardness of materials, the more it will become possible to design hard materials to order, she explains.

However, she points out that in order to prove the theory, single crystals of each material would be needed. So far there are no known ways to isolate or grow such crystals of either material.

The exciting results of the simulations described above remain unconfirmed by experiment, because there's simply not enough of these exotic materials to test. "There has been no report of synthesis of wurtzite boron nitride or hexagonal diamond in large quantity since our 2009 work," says Changfeng Chen.

But the search for ultra-hard crystals continues to turn up promising candidates. Last year, Yongjun Tian of Yanshan University in Qinhuangdao, China, and colleagues studied a cubic form of boron nitride – rather than the hexagonal form Chen studied – which they claimed was also harder than diamond.

The experimental details of that study have been criticised, but Chen believes Tian's material still bears investigating. "The main challenge is to understand the atomistic mechanism underlying this extraordinary result," says Chen.

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Beautiful, expensive and very hard. But now harder minerals have been found to exist (Image: Isifa Image Service sro/Rex Features)